In order to develop long-life and robust power semiconductor modules, high thermal and electrical demands are placed in particular on the upper and lower connection point of the semiconductor (upper side and lower side). The lower side of the semiconductor is usually contacted by means of a solder connection or partly also by means of a sintered or diffusion-soldered connection.
As a standard, the upper side of the semiconductor exhibits a metallization or metal layer that is optimized for the bonding process of thick aluminium wires. Despite the expansion-intensive metallization layers on the upper and lower side of the semiconductor, for reducing electrical losses the semiconductors still become ever thinner. At present, power semiconductors are on the market having a total thickness of only approximately 70 μm.
To increase the reliability of the upper-side contact on the chip it is for example known from DE 10 2011 115 886 A1 to apply a metallic shaped body (copper film) on the chip contacts and to carry out the sintering prior to the wire bonding.
The advantages obtained by the use of metallic shaped bodies to create a connection of a semiconductor chip comprising upper side electrical contacts to thick wires or strips particularly include that neither during bonding nor by preceding or subsequent steps there is a risk of damaging the thin metallisation layers which form the electrical contact structures of the semiconductor. The thermal or mechanical forces involved, for example, in the welding, brazing, sintering or ultrasonic welding of thick wires or strips are at least partially absorbed by the metallic shaped bodies and so reduce the size of such forces transmitted to the semiconductor chip.
Similarly, the metallic shaped bodies may form a mechanical protection of the surfaces, in particular across the electrical contacts of the semiconductor, from frictional contacting test processes (high current tests at wafer level). This permits a safe electrical test of the semiconductor before completing the upper side connection of the semiconductor. For the electrical test, the surface of the metallic shaped body that is bonded to the semiconductor is contacted by special spring tools without risking damage to the fine surface structures of the semiconductor.
In addition an improved current distribution may be achieved. With copper thick wire bonding (for example using up to 600 μm diameter wires), the current now runs from central fixing areas of the wires on the metallic shaped bodies in a distributed manner through the metallic shaped body to the corresponding electrical contacts of the semiconductor.
The metallic shaped body need not extend over the dimensions of the individual semiconductor, but may, in some embodiments, undercut parts of the upper side. In many instances, the size and shapes of metallic shaped bodies may reflect the precise pattern of the electrical contacts of the semiconductor chip. That is to say, the size of the metallic shaped body reflects the size of the electrical contact that it is associated with, and it does not need to extend beyond that area. In other situations, it may extend beyond that area, but not extend beyond the edge of the upper surface of the semiconductor chip.
However, the disadvantage with the procedure described in DE 10 2011 115 886 A1 is that adhesively/cohesively contacting the metallic shaped body by sintering on the wafer that has not yet been separated leads to a strong deformation by bending or warping of the wafer since the differences in the coefficients of thermal expansion of copper (Cu) and silicon (Si) are large and the sintering process takes place at relatively high temperatures (approx. 260° C.) at a relatively high pressure (approx. 10-40 MPa). The thinner the wafer and the thicker the metallic shaped body, the more critical therefore the warpage will be.
In addition, for carrying out the known method also a complicated apparatus is required that has to provide both a high pressure and a high temperature so as to implement the pressure-sintering process on the wafer without at the same time destroying the wafer.
Despite this, in the following processes it is difficult to handle a wafer having a strong warpage.
As early as during the separation a strongly warped wafer will lead to breakages or edge chipping and at the latest during further processing during sintering on the substrate or during subsequent wire bonding, chip breakages on account of the resulting bending stress are very likely.
The alternative, to apply (adhere) the metallic shaped bodies onto the chips in a separate population process would mean that two population processes are necessary, as a result of which the process times and thus the process costs increase. Therefore it would be advantageous if the chip already had a metallic shaped body positioned on the surface during the population on the substrate.